Capacitive type dynamic quantity sensor

ABSTRACT

In a capacitive type dynamic quantity sensor, a width of a beam in a beam portion extending in a direction that is perpendicular to a predetermined deformation direction and a gap disposed between a movable electrode and the fixed electrode in the predetermined deformation direction are approximately identical. Accordingly, manufacturing error is prevented from affecting the sensitivity of the capacitive type dynamic quantity sensor. For example, a manufacturing tolerance error of ±2.5% is allowed as a result of designing the width of the beam and the gap to be identical in length.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of JapanesePatent Application No. 2002-41503 filed on Feb. 19, 2002, the contentsof which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to dynamic quantitysensors, and specifically to a capacitive type dynamic quantity sensorthat detects a dynamic quantity using a capacitance formed between amovable electrode and a fixed electrode.

BACKGROUND OF THE INVENTION

[0003] Conventionally, a capacitive type dynamic quantity sensor such asthe one shown in FIG. 7 is typically constructed by etching a substrate10 such as a semiconductor substrate. The etching forms a trench in thesubstrate 10 to separate a movable portion including beam portions 22and movable electrodes 24 from electrodes of fixed electrode groups 32,42.

[0004] The beam portions 22 extend in a direction perpendicular to the Ydirection in FIG. 7 and are spring-like in operation, as they deform inthe Y direction with respect to a force applied thereon. The movableelectrodes 24 also extend in a direction that is perpendicular to the Ydirection and move in the Y direction along with the beam portions 22.The movable electrodes 24 have, for example, a comb-shapedconfiguration.

[0005] The comb-shaped electrodes of the fixed electrode groups 32, 42are supported by and fixed on the substrate 10 to respectively face themovable electrodes 24.

[0006] According to the above described capacitive type dynamic quantitysensor, a total capacitance CS1 is formed in gaps D disposed between themovable electrodes 24 on the left side in FIG. 7 and the electrodes ofthe fixed electrode group 32, and a total capacitance CS2 is formed ingaps D disposed between the movable electrodes 24 on the right side inFIG. 7 and the electrodes of the fixed electrode groups 42. When aphysical quantity such as acceleration is applied to the capacitive typedynamic quantity sensor, the capacitances CS1, CS2 vary with respect toan amount of the physical quantity. Therefore, the physical quantity isdetected based on the variation of difference between the capacitancesCS1, CS2.

[0007] In the above capacitive type dynamic quantity sensor, the fixedelectrode groups 32, 42 and the movable portion including the beamportions 22 and the movable electrodes 24 are formed at the same time byetching the trench in the substrate 10. Therefore, a manufacturing errorof width B is approximately the same relative to each of the beamportions 22 and the gaps D disposed between the movable electrodes 24and the electrodes of the fixed electrode groups 32, 42. For example, asthe widths B of the beam portions 22 increase in width, the gaps Ddisposed between the movable electrodes 24 and the electrodes of thefixed electrode groups 32, 42 decrease in width.

[0008] Accordingly, the manufacturing error causes variations of thewidths B and the gaps D, and therefore characteristic non-uniformity ofthe capacitive type dynamic quantity sensor becomes large.

[0009] Incidentally, the characteristic non-uniformity of the capacitivetype dynamic quantity sensor can be minimized by enlarging the widths Band the gaps D. However, the capacitances CS1, CS2 consequently decreaseand sensor sensitivity also decreases.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to provide aphysical quantity sensor that is capable of obviating the above problem.

[0011] It is another object of the present invention to provide aphysical quantity sensor that is capable of good sensitivity.

[0012] According to a capacitive type dynamic quantity sensor of thepresent invention, a width of a beam of a beam portion extending in aperpendicular direction relative to a predetermined deformationdirection and a gap disposed between a movable electrode and a fixedelectrode in the predetermined deformation direction are approximatelyidentical.

[0013] Accordingly, the sensitivity of the capacitive type dynamicquantity sensor is not affected. For example, a manufacturing toleranceof ±2.5% in designing the width of the beam and the gap between themovable electrode and the fixed electrode is allowed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other objects, features and advantages of the present inventionwill be understood more fully from the following detailed descriptionmade with reference to the accompanying drawings. In the drawings:

[0015]FIG. 1 shows a plan view of a capacitive type dynamic quantitysensor according to a first embodiment of the present invention;

[0016]FIG. 2 shows a cross sectional view of the capacitive type dynamicquantity sensor taken along line II-II of FIG. 1;

[0017]FIG. 3 shows a cross sectional view of the capacitive type dynamicquantity sensor taken along line III-III of FIG. 1;

[0018] FIG.4 shows an electrical circuit of the capacitive type dynamicquantity sensor according to the first embodiment;

[0019]FIG. 5 shows a schematic view of a relationship between widthvariation ΔD and sensitivity ΔC according to the first embodiment of thepresent invention;

[0020]FIGS. 6A and 6B show beam portions according to a modified versionof the first embodiment; and

[0021]FIG. 7 shows a plan view of a capacitive type dynamic quantitysensor according to a related art capacitive type dynamic quantitysensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] The present invention will be described further with reference tovarious embodiments shown in the drawings.

[0023] (First Embodiment)

[0024] In the present embodiment, a differential capacitance typesemiconductor acceleration sensor (acceleration sensor) S1, or, moregenerally, a capacitive type dynamic quantity sensor is shown.

[0025]FIG. 1 shows a plan view of the acceleration sensor S1. FIGS. 2and 3 show cross sectional views of the acceleration sensor S1 takenalong lines II -II and III-III of FIG. 1. The acceleration sensor S1 is,for example, utilized as a vehicle acceleration sensor or a gyro sensorfor controlling an airbag system, an Antilock Brake System (ABS), a sideskid control system or in any other like system that requires sensing ofa dynamic quantity.

[0026] The acceleration sensor S1 is manufactured on a semiconductorsubstrate using micro-machine technology. Referring to FIGS. 2 and 3, anSOI substrate 10 is used for the semiconductor substrate. The SOIsubstrate 10 includes a first silicon substrate 11, a second siliconsubstrate 12 and an oxide film 13 interposed between the first andsecond silicon substrates 11, 12. The first silicon substrate 11corresponds to a first semiconductor layer, the second silicon substrate12 corresponds to a second semiconductor layer, and the oxide film 13corresponds to an isolation film.

[0027] Referring to FIGS. 1-3, the second substrate 12 has trenches 14in which a configuration referred to collectively as a comb-shapedconfiguration of beams 20-40 including a movable portion 20 and fixedportions 30, 40 is formed. The oxide film 13 includes an opening portion15 in which the comb-shaped configuration of beams 20-40 is formed.

[0028] The movable portion 20 supported across the opening portion 15includes a rectangular plumb portion 21, beam portions 22 and anchorportions 23 a, 23 b. The rectangular plumb portion 21, the beam portions22 and the anchor portions 23 a, 23 b are integrated with each other,and the anchor portion 23 a, 23 b support the plumb portion 21 via thebeam portions 22. As shown in FIG. 3, the anchor portions 23 a, 23 b areformed at peripheral positions of the opening portion 15 of the oxidefilm 13 and are supported by first silicon substrate 11. Therefore, thebeam portions 22 and the plumb portion 21 are disposed above the openingportion 15.

[0029] Each of the beam portions 22 has two beams, both of which extendin a parallel direction and join with each other at end portionsthereof. Accordingly, the beam portions 22 form a rectangular frame anddeform in a direction perpendicular to a longitudinal side of the beams.Specifically, according to the beam portions 22, the plumb portion 21moves in a Y direction (arrow direction in FIG. 1) when accelerationincluding a Y direction component is applied thereto, and returns to aninitial position thereof when the acceleration decreases. That is, themovable portion 20 moves in a deformation direction (i.e., the Ydirection) of the beam portions 22 above the opening portion 15 uponapplication of acceleration.

[0030] The movable portion 20 also includes movable electrode groups 24that extend in a direction perpendicular to the Y direction from bothsides of the plumb portion 21. In FIG. 1, each side of the movableelectrode groups 24 include four electrodes that protrude from right andleft sides of the plumb portion 21, respectively, and respectiveelectrodes of the movable electrode groups 24 are positioned above theopening portion. Accordingly, the movable electrode groups 24 areintegrated with the beam portions 22 and the plumb portion 21 andtherefore move in the Y direction with the beam portions 22 and theplumb portion 21.

[0031] The fixed portions 30, 40 are supported on respective opposingperipheral sides of the opening portion 15 of the oxide film 10, wherethe respective opposing peripheral sides are opposite the sidessupporting the anchor portions 23 a, 23 b. The fixed portions 30, 40include a first fixed portion 30 on a left side of FIG. 1 and a secondfixed portion 40 on right side thereof.

[0032] The fixed portions 30, 40 include respective wiring portions 31,41 and a plurality of respective first and second fixed electrode groups32, 42. The wiring portions 31, 41 are fixed on the first siliconsubstrate 11 at the peripheral portion of the opening portion 15 of theoxide film 10. In FIG. 1, each of the fixed electrode groups 32, 42 isformed by four electrodes. Respective electrodes of the fixed electrodegroups 32, 42 are supported on the wiring portions 31, 41 at endportions thereof and extend in parallel with, and oppose, respectiveelectrodes of the movable electrode groups 24 so as to define respectivepredetermined gaps D therebetween. Hereinafter, the fixed electrodegroup 32 of the first fixed portion 30 will be referred to as a firstfixed electrode group 32, and the fixed electrode group 42 of the secondfixed portion 40 will be referred to as a second fixed electrode group42.

[0033] Fixed electrode pads 31 a, 41 a for wire bonding are formed atpredetermined positions of the wiring portions 31, 41 of the first andsecond fixed portions 30, 40. A movable electrode wiring portion 25 isformed on the anchor 23 b and has a movable electrode pad 25 a at apredetermined position thereof. The pads 25 a, 31 a, 41 a are, forexample, made of aluminum.

[0034] The acceleration sensor S1 is mounted on a package (not shown) ata reverse side of the first silicon substrate 11 corresponding to a sideopposite the oxide film 13 via adhesive. An electrical detection circuit100 (FIG. 4) is included in the package and is electrically connected tothe electrode pads 25 a, 31 a, 41 a via wiring such as gold, aluminum orthe like.

[0035] Manufacture of the acceleration sensor S1 will now be described.A mask (not shown) corresponding to a shape of the comb-shapedconfiguration of beams 20-40 is formed on the second silicon substrate12 of the SOI substrate 10 by photolithography. The trenches 14 areformed on the second substrate 12 by dry etching with CF₄, F₆ or thelike. Accordingly, the comb-shaped configuration of beams 20-40 isformed on the SOI substrate 10. The oxide film 13 is then removed bysacrifice-etching with hydrofluoric acid or the like. Therefore, thecomb-shaped configuration of beams 20-40 is supported by the firstsilicon substrate 11.

[0036] According to the acceleration sensor S1, a total capacitance CS1is formed in gaps D defined in the Y direction between each of themovable electrodes 24 and corresponding ones of the fixed electrodegroup 32, and a total capacitance CS2 is formed in gaps D defined in theY direction between each of the movable electrodes 24 and correspondingones of the fixed electrode group 42. When a physical quantity such asacceleration is applied to the capacitive type dynamic quantity sensor,the capacitances CS1, CS2 vary with respect to an amount of the physicalquantity. Therefore, the physical quantity is detected based on thevariation between the capacitances CS1, CS2.

[0037]FIG. 4 shows a schematic diagram of a detection electrical circuit100 of the present acceleration sensor S1. The detection electricalcircuit 100 includes a switched capacitor circuit (SC circuit) 110having a capacitor 111 with a capacitance Cf, a switch 112 and adifferential amplifier circuit 113. The SC circuit 110 converts an inputcapacitance difference (CS1-CS2) between the capacitances CS1, CS2 to avoltage.

[0038] According to the present acceleration sensor S1, for example, acarrier wave W1 with an amplitude Vcc is applied to the fixed electrodepad 31 a, and a carrier wave W2 with an amplitude Vcc that is invertedwith respect to the carrier wave W1 is applied to the fixed electrodepad 41 a. The switch 113 of the SC circuit 110 is opened and closedbased on a predetermined timing. Therefore, an acceleration applied tothe acceleration sensor S1 is represented as an output voltage V0according to the following formula:

V0=(CS1-CS2)·Vcc/Cf  (1)

[0039] Further, in the present acceleration sensor S1, the gaps Ddefined between each of the movable electrodes 24 and corresponding onesof the fixed electrodes 32, 42 are defined as having the same width asthe widths B of the beam portions 22, with the widths B also beingdefined in the Y direction. Accordingly, it is possible to prevent theacceleration sensor from having decreased sensor characteristics due tomanufacturing error without the need to enlarge the widths D, andtherefore decrease the sensitivity of the acceleration sensor S1.

[0040] Generally, in a capacitive type dynamic quantity sensor,sensitivity varies linearly with capacitance. The variation of thecapacitance corresponding to “ΔC” is expressed as follows, where a totalcapacitance formed between the movable electrodes 24 and the electrodesof fixed electrode groups 32, 42 at which acceleration is zerocorresponds to “Co”, mass of the movable portion 20 corresponds to “m”,and spring constant of the beam portions 22 corresponds to “k”.Incidentally, “D” corresponds to the gaps D as discussed above.

ΔC=(2·Co·m)/(k·D)  (2)

[0041] The sensitivity, i.e., the variation AC of the capacitance,varies based on manufacturing error such as etching error in forming thetrenches 14 and sacrificial-etching error in removing the oxide film 13.The manufacturing error is defined by two types of size non-uniformity,that is, size non-uniformity in a direction parallel to a plane surfaceof the SOI substrate 10 of the beam portions 22, the movable electrodes24, and the electrodes of the fixed electrode groups 32, 42, and sizenon-uniformity in a direction parallel to the thickness of the SOIsubstrate 10. The former corresponds to width non-uniformity ΔD, and thelatter corresponds to thickness non-uniformity Δh.

[0042] The sensitivity ΔC expressed in formula (2) varies as follows,where a thickness of the beam portions 22, the movable electrodes 24,and the electrodes of the fixed electrode groups 32, 42 corresponds to“h”, and widths of the movable electrodes 24 and of the electrodes ofthe fixed electrode groups 32, 42 correspond to “W”. Incidentally, “D”corresponds to the gaps D and “B” corresponds to the widths B asdiscussed above. $\begin{matrix}{{\Delta \quad C} \propto \frac{\frac{\left( {h + {\Delta \quad h}} \right)}{\left( {D - {\Delta \quad D}} \right)} \cdot \left( {h + {\Delta \quad h}} \right) \cdot \left( {W + {\Delta \quad D}} \right)}{\left( {h + {\Delta \quad h}} \right) \cdot \left( {B + {\Delta \quad D}} \right)^{3} \cdot \left( {D - {\Delta \quad D}} \right)}} & {\bullet 3\bullet j}\end{matrix}$

[0043] The formula (3) transforms through the following formulae,resulting finally in formula (6). $\begin{matrix}{{\Delta \quad C} \propto \frac{\left( {h + {\Delta \quad h}} \right) \cdot \left( {W + {\Delta \quad D}} \right)}{\left( {B + {\Delta \quad D}} \right)^{3} \cdot \left( {D - {\Delta \quad D}} \right)^{2}}} & {\bullet 4\bullet j} \\{{\Delta \quad C} \propto \frac{\left( {h + {\Delta \quad h}} \right)}{\left( {B + {\Delta \quad D}} \right)^{2} \cdot \left( {D - {\Delta \quad D}} \right)^{2}}} & {\bullet 5\bullet j} \\{{\Delta \quad C} \propto \frac{\left( {h + {\Delta \quad h}} \right)}{\left\{ {{B \cdot D} + {{\left( {D - B} \right) \cdot \Delta}\quad D} - {\Delta \quad D^{2}}} \right)^{2}}} & {\bullet 6\bullet j}\end{matrix}$

[0044] Referring to formula (6), the denominator has a minimum valuewhen the widths B are equal in size to the gaps D.

[0045]FIG. 5 shows a relationship between the width non-uniformity ΔDand the variation of the capacitance ΔC. In FIG. 5, a solid linerepresents the relationship when the widths B are equal in size to thegaps D, and a dotted line represents the relationship when the widths Bare larger in size than the gaps D. An inflection point of a quadraticcurve illustrated by the solid line is a point of ΔD=0, and that of aquadratic carve illustrated by the dotted line is shifted from the pointof ΔD=0.

[0046] In a manufacturing process, the variation of the capacitance ΔCis shifted from the center point, that is, 0 μm. For example, if ΔD isshifted in a range from −1 μm to +1 μm, non-uniformity ΔΔC1 of thevariation of the capacitance ΔC when the widths B equal the gaps D issmaller than non-uniformity ΔΔC2 of the variation of the capacitance ΔCwhen the widths B are larger than the gaps D.

[0047] Also, it has been shown that the relationship is identical ifnon-uniformity of the variation of the capacitance ΔC when the widths Bequal the gaps D is compared with non-uniformity of the variation of thecapacitance ΔC when the widths B are smaller than the gaps D. Thevariation of the capacitance ΔC has a minimum value when the widths Bequal the gaps D and an inflection point of a quadratic curverepresenting the relationship between the width non-uniformity ΔD andthe variation of the capacitance ΔC is a point of ΔD=0. Accordingly, bydesigning the acceleration sensor S1 to have identical widths B and gapsD, the sensitivity of the acceleration sensor S1 is not affected due tomanufacturing error.

[0048] Therefore, in the present embodiment, the size of the widths Bare the same as the gaps D. This effect is preferably obtained with theacceleration sensor S1 of which the beam portions 22, the movableelectrodes 24 and the electrodes of the fixed electrode groups 32, 42are simultaneously formed on the substrate 10 (the second substrate 12)by forming the trenches 14 with etching.

[0049] Incidentally, an error tolerance of ±2.5% is acceptable indesigning the widths B and the gaps D. This is because a manufacturingerror of ±2.5% may be generated when a mask pattern corresponding to thecomb-shaped configuration of beams 20-40 is manufactured.

[0050] (Modification)

[0051] In the first embodiment, the beam portions 22 can alternativelybe adapted as a repeatedly turned-shaped pattern illustrated in FIG. 6A,or as an L-shaped pattern illustrated in FIG. 6B. In these cases, widthsB correspond to widths of beams extending in a direction that isperpendicular to the Y direction.

[0052] The opening portion 15 may alternatively be formed in the firstsilicon substrate 11 as well as in the oxide film 13. In this case,after the comb-shaped configuration of beams 20-40 is formed in thesecond silicon substrate 1, the first silicon substrate 11 isanisotropically etched and the oxide film 13 is further etched withhydrofluoric acid or the like.

[0053] In the above embodiments, an acceleration sensor is described;however, other capacitive type dynamic quantity sensors such as angularspeed sensor may also be realized in a similar manner.

[0054] While the above description is of the preferred embodiments ofthe present invention, it should be appreciated that the invention maybe modified, altered, or varied without deviating from the scope andfair meaning of the following claims.

What is claimed is:
 1. A capacitive type dynamic quantity sensorcomprising: a beam portion having a beam for deforming in apredetermined deformation direction based on physical force application;a movable electrode formed integrally with the beam portion to movetherewith and extending in a direction perpendicular to thepredetermined deformation direction; and a fixed electrode facing themovable electrode and being separated therefrom in the predetermineddeformation direction by a predetermined gap; wherein a width of thebeam extending in the direction perpendicular to the predetermineddeformation direction and the gap are approximately identical.
 2. Thecapacitive type dynamic quantity sensor of claim 1, wherein the beam ofthe beam portion includes two beams, both of which extend in a paralleldirection and join with each other at end portions thereof, the width ofeach of the beams being approximately the same as a width of the gap. 3.The capacitive type dynamic quantity sensor of claim 1, wherein thewidth of the beam and the gap are defined so that one may be shifted by±2.5% from another.
 4. The capacitive type dynamic quantity sensor ofclaim 1, wherein the capacitive type dynamic quantity sensor is adaptedfor use in one of a one of vehicle acceleration sensor for airbagcontrol, a gyro sensor for airbag control, for ABS control and for sideskid control.
 5. The capacitive type dynamic quantity sensor of claim 1,further comprising: a plumb portion formed integrally with the movableelectrode and the beam portion, wherein the movable electrode includescomb-shaped electrodes located at both sides of the plumb portion; andthe fixed electrode includes comb-shaped first and second fixedelectrodes extending in parallel with, and opposed to respective ones ofthe comb-shaped electrodes of the movable electrode so as to define thepredetermined gap therebetween.
 6. A method of manufacturing acapacitive type dynamic quantity sensor, wherein the capacitive typedynamic quantity sensor has a beam portion with a beam for deforming ina predetermined deformation direction based on physical forceapplication, a movable electrode formed integrally with the beam portionto move therewith and extending in a direction perpendicular to thepredetermined deformation direction, and a fixed electrode facing andbeing separated from the movable electrode, comprising: forming a trenchin a substrate to form the beam portion, the movable electrode and thefixed electrode simultaneously, wherein the forming of the trench formsthe beam so as to have a width extending in the direction perpendicularto the predetermined deformation direction, and the gap disposed betweenthe movable electrode and the fixed electrode so as to have a width inthe predetermined deformation direction that is approximately identicalto the beam width.
 7. The method of claim 6, wherein the forming thetrench includes forming the trench by etching the substrate.
 8. Themethod of claim 6, wherein the forming the trench defines the width ofthe beam and the gap so that one of which may be shifted by ±2.5% fromanother.
 9. A method of manufacturing a capacitive type dynamic quantitysensor comprising: forming a trench in a substrate to form a beamportion, a movable electrode and a fixed electrode simultaneously,wherein the forming of the trench forms the beam so as to have a widthextending in a direction perpendicular to a predetermined deformationdirection, and a gap disposed between the movable electrode and thefixed electrode so as to have a width in the predetermined deformationdirection that is approximately identical to the beam width.